The LDD process utilizes a lightly doped source/drain region that is driven into a substrate adjacent the gate region of a transistor while the heavily doped drain and source regions are laterally displaced away from the gate by use of a sidewall spacer on the gate. Some minor lateral diffusion under the gate of the light dopant may occur but LDDs typically avoid conventional problems with excess lateral or vertical impurity diffusion. LDDs have been developed to reduce hot electron induced device degradation effect in N-channel transistors resulting from short channel lengths. Hot carrier instability is a conventional problem which is the injection of either electrons or holes into the gate resulting from high electrical fields between the source and drain. The problem may be particularly acute near the drain where impact ionization occurs. High energy or hot carriers are injected into the gate dielectric resulting in a combination of threshold voltage shift, mobility degradation and increased series resistance depending upon the type of device structure.
In a conventional LDD process, the lightly doped regions are implanted into the substrate after the gate has been formed but prior to sidewall spacer formation. A dielectric is typically deposited after the gate has been formed and removed from the horizontal surfaces using anisotropic etching. A sidewall spacer having a roughly quarter-circular cross-section is typically formed. A heavy source/drain dopant is then implanted with the gate and sidewall spacer acting as a mask to provide source and drain regions laterally displaced from the gate edges by the thickness of the sidewall spacer. The conventional LDD process requires the use of four photoresist masks, which makes the process expensive and increases the likelihood of errors during manufacture. The conventional LDD process also requires a p- region under the sidewall spacer of a P-channel transistor's gate. Adding the lightly doped drain to P-channel transistors adds extra resistance to the source/drain regions. In general, the LDD process is not very beneficial to P-channel devices since a P-channel transistor is not noticeably degraded by the hot carrier phenomena due to the difference between electron and hole mobility.